1. Field of the Invention
The present invention relates to a process for the removal of asphaltenes of high molecular weight and high softening point from heavy hydrocarbon materials, by using a precipitant to precipitate these asphaltenes and separate them in a solid form.
2. Description of the Prior Art
Heavy crude oils have high asphaltene content which is detrimental to further processing of these crude oils to convert them into more valuable products. This is due to the high metal concentration in the asphaltenes. For example, in catalytic desulfurization, these metals deposit on the catalyst surfaces poisoning the catalyst, and increase significantly the desulfurization cost.
In distilling these heavy oils, it is only possible to recover about 40 to 60 weight percent of distillates and heavy gas oil, still leaving a large fraction of heavy residue with high concentrations of asphaltenes, metals and sulfur. By means of solvent deasphalting using a precipitant of high molecular weight, it is possible to make a deep cut and extract more oil and resins, thus increasing the recovery of oil products almost free of asphaltenes and having a lower metal content, which can be used as a feed to downstream refining processes, such as fluid catalytic cracking, catalytic desulfurization or the like. The metals are mainly concentrated in the precipitated asphaltenes of high softening point, which are discarded.
Paraffinic solvents have been used for many years to deasphalt heavy hydrocarbons. The development of the propane and butane deasphalting processes was a very important contribution to petroleum technology in the refining of residual oils and provided a method for a substantially complete separation of oil from the asphaltic material contained in the residual derived from any crude source. These processes usually operate near the critical temperature of the paraffinic solvent and above the liquefaction temperature of the asphaltenes, and the deasphalting step is performed in a countercurrent solvent extraction tower like the one described in U.S. Pat. No. 3,811,843.
In recent years, solvent deasphalting has evolved in the direction of increasing deasphalted oil yields, using heavier paraffinic solvents, like butane/pentane mixtures or pentanes alone. The process operates with two or more extraction stages, and the heavy materials removed in the process include both resins and asphalt. This procedure is described in U.S. Pat. No. 4,239,616, where asphaltenes, resins and oil are separated in three stages operating at temperatures and pressures above the critical conditions of the solvent used. In this manner, energy is saved since solvent recovery from the oil is effected by the difference in density between the oily phase and the gas dense solvent phase, avoiding evaporation of the solvent from the oil.
A characteristic common to all these deasphalting processes is that the asphaltic materials are obtained in liquid phase after the solvent is recovered by stripping with steam or an inert gas, at varying temperatures depending on the asphalt softening point. Usually, these processes have some limitations if higher molecular weight solvents, like pentane, hexane, heptane or light naphthas are used to give a high yield of oil, since these hydrocarbons selectively precipitate an almost oil-free asphaltenic material of very high softening point. It is well accepted that processing asphaltic material having a softening point higher than 200.degree. C. leads to almost insurmountable difficulties. For example, G.B. Pat. No. 2,031,011A, indicates that in a pentane deasphalting operation, where operating conditions were set to obtain a high yield of deasphalted oil, the resultant asphaltic material, with a softening point of 186.degree. C., caused a blocking phenomenon in the pipes of the apparatus because its viscosity was too high, even at 186.degree. C., rendering the operation impossible. Accordingly, the conclusion was that obtaining high yields of deasphalted oils by such processes is not industrially viable unless a fluxant oil is added to lower the viscosity.
These difficulties in operation with solvents heavier than propane and butane arise from the fact that the precipitated asphaltic material is not oily, but consists of very fine solid asphaltene particles which can easily plug or stick to either the extractor walls or the stripping column. It is well documented in the literature (E. W. Funk, Canadian Journal of Chemical Engineering, Vol. 57, p. 333 (June 1979)), that at room temperature, the particle size of the precipitated asphaltenes can be less than 1-2 microns, and the particle size distribution largely depends on the solvent molecular weight. The pentane asphaltenes are of larger particle size than the hexane asphaltenes and this trend continues through octane, etc. The molecular weight of these asphaltenes also varies, and it has been reported to be in the range of 2000 up to 100,000. At the same time, the softening point is from 170.degree. C. for pentane asphaltenes to over 280.degree. C. for asphaltenes precipitated with naphthas.
To overcome some of these operating difficulties, particular methods have been proposed to separate the fine solid particles of asphaltenes using a centrifugal force field. U.S. Pat. No. 3,159,571 describes a process to separate the asphaltenes and ash-forming constituents from an oil mixture by forcing the solvent-oil mixture through one or more hydrocyclones. The oil mixture is previously heated to promote agglomeration of precipitated particles. Temperatures of 35.degree.-65.degree. C. are applied both for agglomeration and during hydrocyclone separation. Precipitation can also be supplemented by aliphatic polar compounds including alcohols, ethers and ketones.
French Pat. No. 1,576,871 and equivalent G.B. Pat. No. 1,175,028 describe an improved process to separate asphaltenes using one or more hydrocyclones. Asphaltenes are precipitated at a temperature which is 5.degree. to 15.degree. C. below the softening point of the asphaltenes. The process has two separation stages in series to separate oil, resins and asphaltenes. Asphaltenes are separated in two hydrocyclones. The first one separates the oil-free asphaltenes, and in the second the asphaltenes are washed with fresh solvent. The oil is separated from the resins by means of a settling tank, at a temperature 10.degree. to 50.degree. C. below the critical temperature of the mixture. In these patents it is assumed that in the washing step performed in the second cyclone there is a complete separation of the solvent and the solid asphaltenes, and that solvent-free asphaltenes are obtained at the bottom outlet of the second cyclone.
It is well known, however, that hydrocyclones, even operating at optimum conditions, are not able to yield an almost dry solid underflow. In the best cases, the solid concentration never reaches more than 60% by weight. Therefore, these processes have an economic drawback due to the solvent loss in the asphaltenic stream.
In the previously mentioned U.S. Pat. No. 3,159,571, as well as in U.S. Pat. No. 4,101,415, it is indicated that the solvent can be recovered in a conventional stripping column where any residual precipitant is flashed off and recycled into the system. Again, the stripping column will have an efficient operation removing the solvent only if the asphaltenes are in a low viscosity liquid phase. Otherwise, when heated with steam the asphalt will stick to the walls and pipes, plugging the stripper. This, of course, limits application of the process to asphaltenes of medium or relatively low softening points (&lt;180.degree. C.).
To overcome this problem with high softening point asphaltenes, U.S. Pat. No. 3,159,571, suggests admixing the asphaltenes with a suitable diluent in order to discharge the solids as dissolved matter. Clearly, however, the addition of any fluxant is detrimental to the yield of valuable liquid products from the deasphalting operations and it is detrimental to the process economics, since the fluxant, which is of higher value than the asphaltenes, is mixed with them and discharged.
Canadian Pat. No. 842,768 describes an improved process with respect to earlier patents, which uses a hydrocarbon/alcohol mixture as precipitant and in which the separation of the solid asphaltenes is performed in a system of two hydrocyclones. The suspension of solid asphaltene particles leaving the hydrocyclone is passed through a filter where the asphaltenes are retained and after removal of the liquid left in the filter cake, the asphaltenes have a powdery appearance. The filtrate is recycled back to the process as precipitant. Although in this patent it is shown that it is possible to filter the asphaltenes, it is not demonstrated that filtration could be economically attractive in a commercial size plant. Because of the very small size of asphaltene particles, filtration rates would be extremely low and the process would incur very high capital and operating costs. The filter cake would retain a significant amount of solvent that would have to be recovered in an additional stage to avoid expensive solvent losses. This solvent recovery stage is not disclosed in the Canadian patent.
U.S. Pat. No. 4,211,633 discloses a process to separate asphaltic materials from liquified coal or other liquified hydrocarbonaceous materials, using a natural gasoline fraction with a boiling range of 200.degree. to 400.degree. F. as a solvent extraction agent and then effecting a centrifugal separation of the asphaltic fraction at elevated temperatures and pressures. The temperature range is 100.degree.-200.degree. C. and pressure is 2-10 atmospheres. Although it is stated that the resulting separated asphaltic material will have far less heptane soluble material than that obtained by the above-mentioned procedures, this conclusion is based on the assumption that centrifugal equipment that can operate at high pressure and temperature is commercially available. It is well known that large size centrifuges, which can stand these levels of pressure and temperature, are not yet on the market although they are in the development stage.
The process includes only one separation stage using a centrifugal type reactor operating at high pressure and temperature. In this unique centrifuge, the asphaltic fraction is hypothetically withdrawn from the centrifuging zone substantially free of oil and resins and is discharged. However, it is known from present practice in centrifuge technology that currently it is not possible to obtain a practically dry solid from this equipment, and that it must contain at least 20 to 40% by weight of entrained solution. Thus, the solvent losses increase solvent make-up and oil and resins dissolved in the entrained solution decrease the overall yield of oil recovered--all factors which reduce process feasibility.
Also, it is well known in the art that as the extraction temperature increases, the deasphalted-oil yield is reduced. (D. L. Mitchell and J. G. Speight, "The Solubility of Asphaltenes in Hydrocarbon Solvent", FUEL, Vol. 5, 149-52 (April 1973)). Therefore, a larger fraction of asphaltenes would be precipitated if U.S. Pat. No. 4,211,633 were followed. These drawbacks limit the commercial application of the process and make its use on an industrial scale impractical.
U.S. Pat. No. 4,101,415, assigned to Phillips Petroleum Co., relates to a process that combines the traditional liquid-liquid extractor with a second stage liquid-solid separator. It is disclosed that the performance of a counter-current contactor can be improved by using a second separation stage to separate solid asphaltenes. Although the type of equipment used in the liquid-solid separation zone is not disclosed, it is clear that this separation is effected at high pressure (550 psig) and at relatively high temperature. Also, the solvent to oil ratio of 40:1 in this zone is extremely large; consequently, the use of any type of centrifugal separator is precluded due to the extremely severe operating conditions and the large throughput to the separators. Solvent recovery from the asphaltenes is said to be performed in a flash zone without specifying any type of equipment. However, under the operating conditions specified in this zone (5psig and 50.degree. F.), it is clear that the process is preferentially applied to solvents like propane and butane as shown in the typical operation example. In any case, this process is more oriented toward the production of lube oil stock and blended asphalt. It is an improvement of the conventional liquid-liquid extraction processes that use the traditional liquid contactor, and is generally limited to a low or moderate yield of deasphalted oil.
Great Britain Pat. No. 1,340,022 discloses a process for the preparation of aqueous asphaltene suspensions. The suspensions produced include a colloidal clay, which serves as a stabilizer and a nonionic detergent. The suspension is stable for long periods of time and can be used to handle and transport the asphaltene slurry coming out from the hydrocyclone, from which the solvent can be removed by evaporation. The water suspension can be easily transported after the solvent has been removed at a temperature lower than 100.degree. C. The main drawback of this method of handling solvent recovery from the solid asphaltenes is the cost increase due to detergent and stabilizer consumption, since they are discharged with the asphaltenes. Another disadvantage is that the addition of clay can be detrimental in the combustion of the asphaltene suspension, causing damage at the burner tips and producing fouling in the tubes of the boiler, reducing its thermal efficiency. In addition, if this suspension is filtered to recover the detergent-water solution, the filtration process is significantly hindered by the presence of fine clay particles.
Previous processes for separation of solid asphaltenes do not provide a reliable and economic technique to recover the solvent from the asphaltene fraction and have hindered scale-up to commercial application. Solid deasphalting technology has not been available for commercial exploitation in the absence of a practical and economical method to recover the solvent, minimizing entrainment of resins.
Among other disadvantages, high energy costs are incurred in the processes described in the prior art since very diluted asphaltene slurries must be first evaporated and then stripped to completely recover the precipitating solvent.
Also, in prior art evaporation procedures sticking of the asphaltenes to the heating surfaces creates insurmountable operating difficulties making commercial applications unfeasible. The hard asphaltic residue, which normally has a high softening point and contains high concentrations of metals, usually begins to decompose at temperatures well below its softening point, creating insurmountable problems in conventional deasphalting equipment, which operates with butane or pentane, and in which solvent recovery from asphaltenes is done in conventional evaporators.